This invention relates to steam reforming and in particular to the catalytic steam reforming of hydrocarbons. Steam reforming is widely practised and is used to produce hydrogen streams and synthesis gas for a number of processes such as ammonia, methanol and Fischer-Tropsch process for the synthesis of carbon-containing compounds such as higher hydrocarbons. In a steam reforming process, a desulphurised hydrocarbon feedstock, e.g. natural gas or naphtha, is mixed with steam and passed at elevated temperature and pressure over a suitable catalyst, often nickel on a suitable refractory support, e.g. alumina or calcium aluminate cement. The composition of the product gas depends on, inter alia, the proportions of the feedstock components, the pressure and temperature. The product normally contains methane, hydrogen, carbon oxides, steam and any gas, such as nitrogen, that is present in the feed and which is inert under the conditions employed. For applications such as methanol synthesis it is generally desirable that the molar proportions of hydrogen and carbon oxides are such that the value of ([H2]xe2x88x92[CO])/([CO2]+[CO]), where [H2], [CO2] and [CO] are the molar proportions of hydrogen, carbon dioxide and carbon monoxide respectively in the product, is about equal to 2. For applications such as Fischer-Tropsch synthesis, it is desired that the ratio of [H2]/[CO] is about 2 and the amount of carbon dioxide present is small. For other applications it may be desirable to have a [H2]/[CO] ratio below 2.
It has been proposed in U.S. Pat. No. 4,999,133 to make a synthesis gas having a low CO2 content and a [H2]/[CO] ratio below 2.5 from feedstocks such as natural gas by a steam reforming process wherein in a first, primary reforming, stage part of the feedstock is reformed with steam in a fired furnace, the remainder of the feedstock is added to the primary reformed product of the first stage and the resultant mixture subjected to a stage of secondary reforming wherein the feed is subjected to partial combustion by the addition of oxygen and then passed over a secondary reforming catalyst. In an example there is described the production of a gas having a carbon dioxide content of 2.6% (on a dry basis) and a [H2]/[CO] ratio of 2.18 by reforming at a pressure of about 49 atms. abs. with a secondary reformer exit temperature of 1149xc2x0 C. 83% of the feedstock bypasses the first reforming stage and over 0.51 moles of oxygen are added per gram atom of hydrocarbon carbon fed to the reforming stages. Not only is a large proportion of oxygen required, but also the high outlet temperature presents metallurgical problems and the need to introduce the feedstock that has bypassed the first reforming stage into the hot feed to the secondary reforming stage presents engineering difficulties.
It has been proposed in U.S. Pat. No. 4,910,228 to make synthesis gas having the stoichiometric composition for methanol synthesis but having a [H2]/[CO] ratio of about 2.9 by a primary/secondary reforming process using a heat exchange reformer for the primary reforming stage with the heat required for the primary reforming being supplied by the sensible heat in the secondary reformed product.
We have realised that by the use of a heat exchange reformer, the use of such large amounts of oxygen and consequent very high temperatures can be avoided and a product having the desired composition can be achieved if a major proportion, if not all, the carbon dioxide is separated from the secondary reformed product, before or after subsequent processing, and recycled to the feed. With a light feedstock, such as natural gas, a [H2]/[CO] ratio of 2 or below can be achieved even with recycle of substantially all of the carbon dioxide. This enables more synthesis gas to be made from a given amount of feedstock. On the other hand, with alternative processes such as autothermal reforming or partial oxidation, when using a light feedstock in order to obtain a [H2]/[CO] ratio of 2 some carbon dioxide has to be exported.
It has been proposed in GB 2 179 366 to produce a gas stream having a [H2]/[CO] value of about 2 using such a heat exchange reformer with recycle of carbon dioxide separated from the product. In the process of that reference part of the feedstock is fed to the primary, i.e. heat exchange, reformer while the remainder is fed to the directly to the secondary reformer. In the examples, the process was operated under conditions such that the product reformed gas (before carbon dioxide separation) had a relatively high carbon dioxide content, ranging from about 27% to about 42% by volume on a dry basis. The amount of carbon dioxide that was separated and recycled was thus large, and exceeded the amount of feedstock on a molar basis. The separation and recycle of such a large amount of carbon dioxide would render such a process uneconomic.
We have realised that such a process can be operated to produce a reformed gas having a significantly lower carbon dioxide content, thus obviating the need for separation and recycle of such large amounts of carbon dioxide.
Accordingly the present invention provides a process for the production of a gas for use in the synthesis of carbon-containing compounds comprising the steps of:
a) subjecting a gaseous mixture containing at least one hydrocarbon and 0.6 to 2 moles of steam per gram atom of hydrocarbon carbon in said mixture to catalytic primary reforming at an elevated temperature in a heat exchange reformer by passing said mixture through tubes containing a steam reforming catalyst heated by a hot gas stream flowing past the exterior of said tubes, thereby forming a primary reformed gas stream;
b) without the addition of further hydrocarbon, subjecting the primary reformed gas stream to secondary reforming wherein it is subjected to partial combustion with a gas containing free oxygen to form a hot partially combusted gas stream which is then passed through a bed of a secondary reforming catalyst, thereby forming a secondary reformed gas stream;
c) passing said secondary reformed gas stream past the exterior of the tubes of the heat exchange reformer as said hot gas stream, thereby heating the gas inside said tubes and cooling said secondary reformed gas stream;
d) further cooling said secondary reformed gas stream, condensing steam therefrom, and separating out the condensed water to give a de-watered secondary reformed gas stream, the reforming conditions being selected to give a de-watered secondary reformed gas stream having a carbon dioxide content below 20% by volume;
e) recovering carbon dioxide from said de-watered secondary reformed gas stream, before or after use of the latter for synthesis of the carbon-containing compounds, and
f) adding the recovered carbon dioxide to the gaseous mixture before the latter is fed to the heat exchange reformer, the amount of recycled carbon dioxide being 0.2 to 0.6 moles per gram atom of hydrocarbon carbon in the gaseous mixture.
In the present invention, the primary reforming is effected using a heat exchange reformer. In one type of heat exchange reformer, the catalyst is disposed in tubes extending between a pair of tube sheets through a heat exchange zone. Reactants are fed to a zone above the upper tube sheet and pass through the tubes and into a zone beneath the lower tube sheet. The heating medium is passed through the zone between the two tube sheets. Heat exchange reformers of this type are described in GB 1 578 270 and WO 97/05 947.
Another type of heat exchange reformer that may be used is a double-tube heat exchange reformer as described in aforesaid U.S. Pat. No. 4,910,228 wherein the reformer tubes each comprise an outer tube having a closed end and an inner tube disposed concentrically within the outer tube and communicating with the annular space between the inner and outer tubes at the closed end of the outer tube with the steam reforming catalyst disposed in said annular space. The external surface of the outer tubes is heated by the secondary reformed gas. The mixture of hydrocarbon feedstock, carbon dioxide and steam is fed to the end of the outer tubes remote from said closed end so that the mixture passes through said annular space and undergoes steam reforming and then passes through the inner tube. As in the double-tube reformer of the aforesaid U.S. Pat. No. 4,910,228, in the present invention preferably insulation is provided on the walls of the inner tube.
In the process of the invention a gaseous mixture is formed from a hydrocarbon feedstock, steam and the recycled carbon dioxide, and has a steam ratio of 0.6 to 2, i.e. it contains 0.6 to 2 moles of steam per gram atom of hydrocarbon carbon, and a carbon dioxide ratio of 0.2 to 0.6, i.e. it contains 0.2 to 0.6 moles of recycled carbon dioxide per gram atom of hydrocarbon carbon. As will be described, in some cases, the recycled carbon dioxide may contain hydrocarbons and so the carbon content of any hydrocarbons recycled with the carbon dioxide has to be added to that of the feedstock when computing the steam and carbon dioxide ratios.
The hydrocarbon feedstock may be any gaseous or low boiling hydrocarbon feedstock such as natural gas or naphtha. It is preferably methane or natural gas containing a substantial proportion, e.g. over 90% v/v methane. The feedstock is preferably compressed to a pressure in the range 20-100 bar abs. If the feedstock contains sulphur compounds, before, or preferably after, compression the feedstock is subjected to desulphurisation, e.g. hydro-desulphurisation and absorption of hydrogen sulphide using a suitable absorbent, e.g. a zinc oxide bed. Usually it is desirable to incorporate a hydrogen-containing gas into the feedstock prior to hydro-desulphurisation: a part of the de-watered secondary reformed gas can be used as the hydrogen-containing gas. Alternatively, where the recycled carbon dioxide also contains hydrogen, this may be used as the hydrogen-containing gas.
Before, or preferably after, compression of the feedstock, steam is mixed with the feedstock: this steam introduction may be effected by direct injection of steam and/or by saturation of the feedstock, or a mixture thereof with the recycled carbon dioxide, by contact with a stream of heated water. The amount of steam is preferably minimised as this leads to a more efficient process. It is preferred that the steam ratio is below 1.8.
In the process of the invention carbon dioxide is recovered and recycled from the de-watered secondary reformed gas, before or after the latter is used for synthesis. Preferably the amount of recycled carbon dioxide is at least 60%, particularly at least 90%, of the carbon dioxide in the de-watered secondary reformed gas. The recycled carbon dioxide may be incorporated before or, preferably, after mixing the steam with the hydrocarbon feedstock.
The resultant feedstock/steam/carbon dioxide mixture is then subjected to reforming. Before it is fed to the heat exchange reformer, the feedstock/steam mixture, before or after addition of the recycled carbon dioxide, may be subjected to a step of adiabatic low temperature reforming. In such a process, the hydrocarbon/steam mixture is heated, typically to a temperature in the range 400-600xc2x0 C., and then passed adiabatically through a bed of a suitable catalyst, usually a catalyst having a high nickel content, for example above 40% by weight. During such an adiabatic low temperature reforming step any hydrocarbons higher than methane react with steam to give a mixture of methane, carbon oxides and hydrogen. The use of such an adiabatic reforming step, commonly termed pre-reforming, is desirable to ensure that the feed to the heat exchange reformer contains no hydrocarbons higher than methane and also contains a significant amount of hydrogen. This is desirable in order to minimise the risk of carbon formation on the catalyst in the heat-exchange reformer. Accordingly, where the recycled carbon dioxide also contains higher hydrocarbons, as in the case of a preferred embodiment of the invention, the gaseous mixture is preferably subjected to such a pre-reforming step and the recycled carbon dioxide is added prior to such a pre-reforming step. Where the recycled carbon dioxide does not contain higher hydrocarbons, it is preferably added to the gaseous mixture after any pre-reforming step.
After any such pre-reforming step, the mixture is further heated, if necessary, to the heat exchange reformer inlet temperature which is typically in the range 400-550xc2x0 C. During passage through the reforming catalyst, which is usually nickel supported on a refractory support such as rings or pellets of calcium aluminate cement, alumina, titania, zirconia and the like, the endothermic reforming reaction takes place with the neat required for the reaction being supplied from the secondary reformed gas flowing past the exterior surface of the outer tubes. The temperature of the secondary reformed gas is preferably sufficient that the gas undergoing primary reforming in the annular space leaves the catalyst at a temperature in the range 650-800xc2x0 C.
The primary reformed gas is then subjected to secondary reforming by adding a gas containing free oxygen, effecting partial combustion and passing the partially combusted gas through a secondary reforming catalyst. The secondary reforming catalyst is usually nickel supported on a refractory support such as rings or pellets of calcium aluminate cement, alumina, titania, zirconia and the like. The gas containing free oxygen is preferably substantially pure oxygen, e.g. oxygen containing less than 1% nitrogen. However where the presence of substantial amounts of inerts is permissible, the gas containing free oxygen may be air or enriched air. Where the gas containing free oxygen is substantially pure oxygen, for metallurgical reasons it is preferably fed to the secondary reformer at a temperature below about 250xc2x0 C. The amount of oxygen-containing gas added is preferably such that 0.4 to 0.6 moles of oxygen are added per gram atom of hydrocarbon carbon fed to the primary reforming stage. Preferably the amount of oxygen added is such that the secondary reformed gas leaves the secondary reforming catalyst at a temperature in the range 800-1050xc2x0 C. For a given feedstock/steam/carbon dioxide mixture, amount and composition of the oxygen-containing gas and reforming pressure, this temperature largely determines the composition of the secondary reformed gas.
The secondary reformed gas is then used to provide the heat required for the primary reforming step by using the secondary reformed gas as the hot gas flowing past the tubes of the heat exchange reformer. During this heat exchange the secondary reformed gas cools by transferring heat to the gas undergoing primary reforming. Preferably the secondary reformed gas cools by several hundred xc2x0 C. but of course it will leave the heat exchange reformer at a temperature somewhat above the temperature at which the feedstock/steam/carbon dioxide mixture is fed to the heat exchange reformer. Preferably the secondary reformed gas leaves the heat exchange reformer at a temperature in the range 500-650xc2x0 C.
Under some operating conditions there is a risk of corrosion of the heat exchange reactor by the high temperature secondary reformed gas: such corrosion is believed to result from reaction of the carbon monoxide in the secondary reformed gas with iron in the metals employed for the heat exchange reformer construction. Where the reaction conditions are such that such a risk may occur, the iron may be passivated by introducing a sulphur compound, such as dimethyl disulphide into the secondary reformed gas after it leaves the secondary reforming catalyst and before it enters the heat exchange reformer. The amount of such sulphur compound necessary to obviate such corrosion problems is typically such as to give a sulphur content of 0.2-20 ppm by volume in the secondary reformed gas.
After leaving the heat exchange reformer, the secondary reformed gas is then further cooled. Heat recovered during this cooling may be employed for reactants pre-heating and/or for heating water used to provide the steam employed in the primary reforming step. As described hereinafter, the recovered heat may additionally, or alternatively, be used in a carbon dioxide separation step.
Where a sulphur compound has been added to the secondary reformed gas as mentioned above, and the final product is to be used in reactions employing catalysts that are poisoned by sulphur compounds, the sulphur compounds may be removed from the secondary reformed gas after some cooling thereof. For example the sulphur compounds may be removed by passing the secondary reformed gas through a bed of a suitable absorbent for sulphur compounds, such as zinc oxide. Preferably two beds of absorbent are employed in series with suitable flow connections to enable one bed to be taken off-line temporarily and replenished with fresh absorbent without shutting down the reforming process.
The secondary reformed gas is cooled to a temperature below the dew point of the steam in the secondary reformed gas so that the steam condenses. The condensed steam is then separated. The cooling to effect condensation of the steam may be effected by contacting the secondary reformed gas with a stream of cold water: as a result a stream of heated water is formed which may be used to supply some or all of the steam required for reforming.
After separation of the condensed water, the de-watered secondary reformed gas typically contains 5 to 15% by volume of carbon dioxide. In one embodiment of the invention, the de-watered secondary reformed gas is then subjected to a carbon dioxide separation step. This may a conventional xe2x80x9cwetxe2x80x9d process wherein the de-watered secondary reformed gas is contacted with a stream of a suitable absorbent liquid, such as an amine, particularly methyl diethanolamine (MDEA) solution so that the carbon dioxide is absorbed by the liquid to give a laden absorbent liquid and a gas stream having a decreased content of carbon dioxide. The laden absorbent liquid is then regenerated, for example by heating, to desorb the carbon dioxide and to give a regenerated absorbent liquid, which is then recycled to the carbon dioxide absorption stage. At least part of the desorbed carbon-dioxide is recycled to the gaseous mixture subjected reforming step as described above. If the carbon dioxide separation step is operated as a single pressure process, i.e. essentially the same pressure is employed in the absorption and regeneration steps, only a little recompression of the recycled carbon dioxide will be required. Unless it is desired that the product synthesis gas has a very low carbon dioxide content, it is generally not necessary to effect the regeneration of the absorbent liquid to a very low carbon dioxide content. Preferably at least 75%, particularly at least 90%, of the carbon dioxide in the secondary reformed gas is separated in the carbon dioxide separation stage.
Instead of using a xe2x80x9cwetxe2x80x9d carbon dioxide removal stage, a pressure swing adsorption process may be employed.
Before use for the synthesis of carbon-containing compounds, some of the hydrogen may be separated from the de-watered secondary reformed gas if desired in order to modify the [H2]/[CO] ratio. This hydrogen separation may be effected at the same time as carbon dioxide separation, e.g. by using a pressure-swing adsorption process, or it may be separated in a separate step using a membrane separation unit.